Main concern for the short term forecast period will be the developing winter storm which is expected to bring snow to the region as early as Saturday afternoon.

In the meantime, today and tonight will be relatively quiet with mostly cloudy skies and unseasonably low temperatures. Highs today will be in the lower 20s with lows tonight in the middle teens. With winds of 5 to 10 mph overnight, wind chill readings will be in the single digits.

As for the upcoming snow event, the longer range model guidance, with the continued exception of the NAM, are in relatively good agreement. Timing is still a bit tricky as the upper level pattern is evolving into relatively zonal flow and timing of individual shortwaves can be difficult. However, the GFS/European model (ecmwf) are in relatively good agreement on the timing and strength of individual waves, so confidence is increasing on the details and impacts of this system. There should be 2 relatively different phases to the pcpn. Initially, snow should begin to move into the wrn portions of the County Warning Area by early Saturday afternoon. Conditions should still be sufficiently cold through a deep layer of the atmosphere that pcpn type is not in question and will begin as snow. The initial wave of pcpn will be driven by large scale warm advection in advance of a developing sfc trough over the wrn plains. As sfc high pressure settles off the Carolina coast, the Gulf of Mexico will become more open, with deep layer moisture streaming nwd ahead of the trough. Upper level support should be somewhat weak as sheared out shortwaves track across the upper Great Lakes. From Saturday afternoon through early Sunday, the heaviest snow should be over the northern portions of the cwa, generally north of the I-80 corridor. Given the cold in situ conditions, snow to liquid ratios should be a little higher than typical, around 17:1. Current indicators suggest that 4 to 6 inches may fall by early Sunday morning north of I-80 with amounts diminishing rapidly to the south.

By Sunday afternoon, pcpn type becomes more of a concern. Sfc low pressure developing over the plains should approach the middle Mississippi Valley by mid day Sunday, further increasing the warm/moist advection. Sfc temperatures will begin to creep upward through the day, but with warmer air aloft overspreading a colder sfc, where temps should still be at or slightly below freezing. This will set up the potential for a at least a mix of fzra/ra or fzra/sn and a narrow corridor of freezing rain. Timing of the mix pcpn phase of the system remains the biggest uncertainty. The latest guidance would suggest that pcpn could be all snow into early afternoon with the changeover to mixed pcpn and then to liquid occurring over the srn portions of the County Warning Area through the afternoon and into the evening hours. The sfc temperature trend should be atypical as well, with temperatures continuing to climb through the evening. At this point, it looks like the freezing rain could reach as far north as the I-80 corridor through the evening hours, while pcpn over the southern portions of the County Warning Area becomes all liquid. The northern portions of the cwa, especially north of the I-88 corridor will likely remain all snow, though with the warmer air spreading north, liquid to snow ratios should decrease, dropping to 10-12:1. The greatest uncertainty in timing of the evolution increases through Sunday night as the European model (ecmwf) trends a bit faster than the GFS in lifting the sfc low across nrn Illinois and to the ern Great Lakes. This appears to be a result of the GFS showing a bit more phasing of a middle stream shortwave across nrn Illinois and a nrn stream shortwave tracking across nrn Wisconsin. Given the relatively fast zonal flow aloft initially, would tend to favor the faster solution of the ECMWF, which would ultimately lead to pcpn tapering off a bit quicker than the slower, more phased GFS solution. But at this time range, the differences between these 2 solutions would ultimately only cause minor differences in eventual snow totals. Based on the currently favored solution, the northern portions of the County Warning Area could see an additional 3 to 5 inches of snow, with snow amounts, again, rapidly dropping off to the south. For the entire event, currently expect 7 to 10 inches of snow north of the I-80 corridor, with the highest amounts closer to the Wisconsin border. The far southern portions of the County Warning Area may only see 1 to 2 inches.

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Long term... 306 am CST

Monday through Friday...

Dangerously cold temperatures will be the main forecast story next week with sub-zero conditions possible as we head through midweek. Area of low pressure that will bring our snowy conditions over the weekend is progged to lift across the eastern Great Lakes to New England on Monday. GFS spits outs some light quantitative precipitation forecast early in the day despite forecast soundings indicating quickly drying mid levels with subsidence associated with building upper ridge overspreading the area in the afternoon. Could be some spotty light flurries or possibly even drizzle given the shallow depth of the moisture which eventually tops out in the -8 to -10c range for ice nucleation, but in general think the models may be overdoing quantitative precipitation forecast for Monday given the shallow moisture and weak forcing.

Meanwhile on Monday, a strong jet topping an anomalously strong upper ridge over northern Alaska into the Arctic Ocean will help dislodge an extremely cold Arctic airmass and push near -40 850mb temps across the Canadian prairies on Tuesday with -20c to -30c temps overspreading portions of the upper Midwest Wednesday and Thursday. Highs around 30f on Monday will give way to single digit or teens for lows Wednesday morning behind the Arctic front with little recovery during the day Wednesday. Models continue to show some spread in exactly how cold we get, with the latest forecast remaining conservatively on the warmer side of solution envelope mainly due to the influence of warmer MOS guidance. Raw output from the latest runs of the GFS and European model (ecmwf) indicate the possibility of sub zero highs on Thursday over some if not much of the County Warning Area with widespread sub-zero conditions both Wednesday night and Thursday night. In addition, breezy conditions in advance of a strong 1040mb high building into the northern plains Wednesday and Thursday will result in (again conservative) wind chill values of 10 to 20 below.

Deubelbeiss

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Aviation... for the 12z tafs...

Primarily VFR conditions and west winds will prevail through the forecast period. Early this morning, expect variable VFR cigs from 035-050 with an occasional flurry possible, though won't amount to anything more than a few flakes falling from the sky. Better snowfall is expected east of the terminals in northwest in due to lake effect snow. MVFR cigs will be in place at gyy because of this, but expect the heavier snowfall to stay east of the terminal. West winds around 10 kt will continue across all the northern Illinois and northwest in terminals through today and should drop to under 10 kt tonight. Bmd

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Marine... 306 am CST

Moderate northwest winds gusting to near 30 kts are in place early this morning across Lake Michigan but should very gradually ease into the 15 to 25 kt range through the day as high pressure builds towards the region. A Small Craft Advisory remains in effect through late this morning for the Illinois nearshore waters, and through the afternoon into the early evening for the Indiana side when waves should finally diminish below criteria. Low pressure is expected to develop over the Central Plains Saturday night into Sunday turning winds southerly across Lake Michigan, then the low is expected to lift across Southern Lake Michigan Sunday night with north to northwest flow overspreading the lake behind the low Monday. Another cold front associated with a low over Canada will push across the lake on Tuesday with a bitterly cold airmass moving in behind the front. West to northwest gales are possible especially Wednesday into Wednesday night and there is also a concern for freezing spray.